This invention relates to the field of measurement of lead lengths of semiconductor devices and more particularly to the measurement of the relative length of leads of a PLCC (Plastic Leaded Chip Carrier) device to determine if the length of each lead is within a selected tolerance of the length of every other lead.
The PLCC package configuration 10 of FIG. 1 for semiconductor devices is a recent innovation wherein the device includes leads 13 on all four sides of the package 12. This is done to increase the package density on the circuit board, or, in other words, to minimize the amount of motherboard real estate that is required to execute an overall instrument. PLCC packages are typically square, however there are a few rectangular configurations, with each side having four to thirty-one pins.
Each lead 13 of a PLCC package 10 is bent downward along the side of package 12 and then bent back under the package in the shape of the letter "J" as shown in FIG. 2. (some manufactures use "gull wing" shaped leads as apposed to "J" shaped leads) These devices are installed on a circuit board by soldering the leads directly to conductive pads at the ends of traces on the circuit board without the leads passing through the board. To accomplish this the leads must each be of a length that is within a selected tolerance of each other to insure that all of the leads will be soldered to the circuit board during the soldering operation. The lead length tolerance that is currently required in the industry is 0.004 inches or less.
At the present time very elaborate and expensive means are used to measure the lead length of each lead on a PLCC package before it is soldered in place on a circuit board. Those means include microprocessor controled laser measurement systems that measure the actual length of each lead and then compares those lengths with each other to determine if any one or more of the leads is out of tolerance. There are two draw-backs to the use of these devices, they are expensive to purchase and slow to utilize. They provide unnessary information for those units which are within tolerance which are typically the majority of the units produced. The laser measurement systems are indeed overkill for this application.
What is needed is a method and apparatus that is easy to use, provides the necessary yes/no test for identifing the PLCC units with what ever shaped leads that are within tolerance, and is relatively inexpensive to implement and use. The present invention is such a method and apparatus.